DONETSK, Ukraine, July 20 (Reuters) - A leader of separatists said on Sunday they were keeping what they presumed to be the black boxes from the downed Malaysian airliner in the eastern city of Donetsk, but would need experts to confirm that they were the plane's flight recorders.

Ukrainian officials had feared that the black boxes, which when opened could offer an insight into the last moments of the flight, may be handed over to Russia or tampered with by the rebels who Kiev says are trying to destroy evidence of their, and Moscow's, involvement in the downing of the plane.

Moscow and the pro-Russian rebels deny playing any role in the disaster, which killed all 298 people on the plane.

"Some items, presumably the black boxes, were found, and they have been delivered to Donetsk and they are under our control," Aleksander Borodai, prime minister of the self-styled Donetsk People's Republic, told a news conference.

"There are no specialists among us who could pinpoint the look of the black boxes, but we brought to Donetsk some technical items which could be the black boxes of the airliner."

He said they would be handed over to international experts, when they arrived, and reiterated that the rebels did not have the technical capability to hit a plane flying at an altitude of more than 10,000 metres.

Another rebel official, Sergei Kavtaradze, said he thought there were two black boxes, which are painted bright orange despite their name.

Ukrainian Deputy Prime Minister Volodymyr Groysman said he could not comment, but that Kiev had long suspected that the boxes were in rebel hands.

Reuters first reported rescue workers finding a flight recorder on Friday, the second such device after rebels said on Thursday they had found one. The rebels later denied they had the black boxes.

Reuters video news filmed a rescue worker on Friday taking the flight recorder but did not immediately transmit the pictures due to difficulties in relaying film from a hostile area and the need to verify what the film depicted. (Reporting by Lina Kushch; Writing by Elizabeth Piper; Editing by Dale Hudson)